a) Field of the Invention
This invention relates to an X-ray detector.
b) Description of the Prior Art
Recently, a great development of X-ray analyzers has taken place, to which X-ray light sources for laboratories such as small-sized synchrotons and laser plasma X-ray light sources are applied. In keeping with this development, there is a growing demand that the spectra of the X-ray light source should be easily monitored. There is, however, a limit to the use of such detectors that wavelength selection and intensity measurement can be together performed without using any specific spectroscope. For the detector which has been practically used in the region of soft X rays (whose wavelengths are less than 100 .ANG. and relatively long) in particular, an X-ray diode (which will be hereinafter abbreviated to XRD) (refer to KMSF Application Notes, An-3, XRD Filter Design X-Ray Diodes, pages 1 and 3-8 is merely available.
The XRD is adapted to utilize fundamentally a photoelectric effect produced when X rays are incident on a substance. FIG. 1 shows its conceptional view as is also shown in R. H. Day, et al., "Photoelectric quantum efficiencies and filter window absorption coefficients from 20 eV to 10 keV", J. Appl. Phys. 52(11), November 1981. According to FIG. 1, the XRD comprises a cathode 1 made of aluminum or the like and an anode 2 of a wire mesh shape, in front of which an X-ray filter 3 is placed. In a case where a potential difference is generated between the cathode 1 and the anode 2 by a bias power source 4, when X rays are incident on the cathode 1 through the X-ray filter 3, photoelectrons are produced in response to the quantum efficiency of a substance constitutiting the cathode 1. Then, the photoelectrons flow as an electric current through the anode 2, so that if the amount of current flow or electric charge is measured by a measuring device 5 such as an oscilloscope, the amount of light of X rays can be monitored. The reason why the XRD permits also the wavelength selection is that each of the cathode 1 and the X-ray filter 3 has its own quantum efficiency and transmittance. FIG. 2 shows the relationship between the photon energy of incident X rays and the amount of electric charge produced per unit X-ray energy (namely, the quantum efficiency) in the Al cathode 1 (refer to KMSF Application Notes, AN-3, XRD Filter Design, X-ray Diodes, pages 1 and 3-8 and KMSF Application Notes, AN-4, Standard KMSF Filters, pages 1-10).
FIG. 3, on the other hand, shows the relationship between the photon energy of X rays and the X-ray transmittance of the X-ray filter 3 made of carbon having a thickness of 0.1 .mu.m. According to FIG. 2, the Al cathode 1 serves as a low-pass filter with respect to the photon energy of X rays since the quantum efficiency tends to diminish as the photon energy increases. According to FIG. 3, in contrast to this, the X-ray filter 3 assumes the role of a high-pass filter. Therefore, it follows from this that the X rays to be detected traverse both the low-pass filter and the high-pass filter due to the construction of the XRD and the X rays of a certain particular photon energy band can be detected. FIGS. 4A and 4B depict its actual examples. Specifically, FIG. 4A shows the quantum efficiency (the number of electrons produced in the detecting system by an individual photon) in the case of the use of the Al cathode 1, the X-ray filter configured by a Pb filter 0.3 .mu.m thick and an N-Parylene filter 0.1 .mu.m thick put together (refer to KMSF Application Notes, AN-4, Standard KMSF Filters, pages 1-10), and the anode 2 with an Ni mesh.
Further, FIG. 4B shows the quantum efficiency in the case of the use of the Al cathode 1, the X-ray filter configured by the Pb filter 0.3 .mu.m thick, the N-Parylene filter 0.1 .mu.m thick, and a Be filter 0.6 .mu.m thick put together (refer to KMSF Application Notes, AN-4Standard KMSF Filters, pages 1-10 , and the anode 2 with the Ni mesh. According to FIGS. 4A and 4B, it is noted that a band-pass is formed between 2 keV and 3 keV of the photon energy.
The detector of the XRD, by the way, has difficulties in the following two points, though simple in structure.
(1) In FIG. 1, it is required that a bias voltage of more than several hundred volts is applied between the anode 2 and the cathod 1, so that problems arise that its power source system comes to a large scale with a complicated peripheral circuit.
(2) As shown in FIG. 2, the quantum efficiency of the Al cathode 1 is low, so that problems are encountered that the entire quantum efficiency is reduced to nearly 0.01 and consequently the value of S/N will be diminished when a weak signal is detected.